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A one-parameter family of self-adjoint Fredholm operators has a well-known integer-valued invariant, the spectral flow. It counts with signs the number of operators’ eigenvalues passing through zero with the change of parameter. For loops of elliptic operators on a closed manifold, the spectral flow was computed by Atiyah, Patodi, and Singer (1976) in terms of topological data of a loop. But if a manifold has non-empty boundary, then boundary conditions come into play, and situation becomes more complicated. In the talk I will explain how to compute the spectral flow for loops of Dirac type operators with classical boundary conditions in two-dimensional case (that is, for a compact surface with boundary). This result has applications to the Aharonov-Bohm effect for a graphene sheet with holes.

More generally, if operators and boundary conditions are parametrized by points of a compact space X, then the relevant invariant takes values in the odd K-group of X and is called the analytical index. I will show how this index is computed in terms of the topological data of the family over the boundary.

The talk is based on my papers arXiv:1108.0806, 1703.06105, and 1809.04353.